Aircraft vibration flight testing



Dec; 15, 1942. E. E. MINOR x-rxl.vr

AIRCRAFT VIBFATION FLIGHT TESTING l1 Sheets-Sheet 1 Filed July 23, 1940 Dec. 15 1942.v E. E. MINOR Erm. 2,305,263

AA:[RCRAF'I' VIBRATION FLIGHT TESTING 11 sheets-sheet 2` Filed July 23, 1940 Dec. l5, 1942. v E. E. MINOR ETAL 2,305,268

AIRCRAFT VIBRATION FLIGHT TESTING Filed July 23, 1940 1l Sheets-Sheet 3 Dec. 15, 1942. E. E. MINOR ETAL AIRCRAFT VIBRATION FLIGHT TESTING Filedl July 25, 1940 1l Sheets-Sheet 4 Bec. 15, 1942. E, EMlNQR 'g1-AL 2,305,268

AIRCRAFT VIBRATION FLIGHT TESTINGV Filed July as, 1940 11 sheets-sheet 5 Dec. 15, 1942. E. E. MINOR TAL 2,305,268

I AIRCRAFT VIBRATION FLIGHT TESTING Filed July 23, 1940 l1 Sheetsf-Sheet 6 @www Dec. 15, 1942. E. E.. MINOR ETAL Y AIRCRAFT VIBRATION FLIGHT TESTING Filed July 23, 1940 11 Sheets-Sheet 7 Dec. 15, 1942. E. E. MINOR ETAL,

AIRCRAFT VIBRATION FLIGHT TESTING Filed July 25, y.1940 11 Sheets-Sheet 8 fqlllylltll' @Inu Dec. 15, 1942. E E M'INOR -FAL 2,305,268- Y AIRCRAFT VIBRATION FLIGHT TESTING Filed July 2.3, 1940 11 Sheets-Sheet 9 Dec. 15, 1942. E.y E. MlNoR Erm.

AIRCRAFT VIBRATION FLIGHT TESTING Filed July 23, 1940 11 Shee'lZS-Sheel'I 10 A Dec. l5, 1942. E. E. MINOR ETAL 2,305,268

AIRCRAFT VIBRATION FLIGHT TESTING Filed July 23, 1940 l1 Sheets-Sheet ll Patented Dec. 15, 1942 UNITED STATES PATENT OFFICE 2.305.268 A Amcam vlnnuloN FLIGHT TESTING Edward E. Minor, Baltimore; and stanley A. Ku-

patrick, Raspeburg, Md., assignors to The Glenn L. Martin Company, Baltimore, Md..

Appllcatlon July z3, 1940. serial No. 346,906

10 Claims.

The invention relates to a measuring method, and particularly to a method for determining the flutter or other vibratory characteristics of aircraft.

This application is a continuation-in-part of our prior applications Serial Number 214,562, led June- 18, 1938, and Serial Number 288,530, led August 5, 1939.

In the -building of aircraft, it is generally impossible to predict accurately from mathematical calculations, the occurrence of structural vibrations of dangerous amplitude, within the desired operating speed range of the aircraft, due to the inherent characteristics of the aircraft while in free flight. Since the total effect of theselforces can be detected only in actual flight, some part of th plane during testing often begins to ilutter dangerously, and in many cases to a degreeor for a. time sufcient to cause breaking, which usually resultsin the loss of the"aircraft and crew.

Flutter in an aircraft is dependent on the speed thereof with respect to the relative wind. Thus for any part such as a control surface, wing, or other external structural appendage there may exist a critical speed at which the type of self-induced structural vibration of large amplitude known as flutter may occur. This speed may be only-slightly greater than a speed'which is perfectly safe, and at which no self-induced vibration occurs. Thus as the speed of the ship is gradually increased during testing, it may suddenly reach the critical speed with the resultV that parts may begin to. vibrate dangerously Awith practically no fore-warning,

The primary object'of the present invention is to provide a method by which an aircraft may be tested and its flutter characteristics determined without actually producing dangerous utter. Particularly, the invention contemplates the Y accurate determination, for any part, of the critical flutter speed therefor, without ever bringing the aircraft up to this speed. i

Still another object of the invention is to make it 'possible to determine this critical speed while the aircraft is in flight so -that in a single flight the desired information can be obtained by gradllally stepping up the speed of the ship to a value below the criticalspeed at which the desired result may be determined. v

Still another object of the invention is to make it possible to determine this critical speed in flight so that the desired information can b e obtained for design changes to increase the critical speed to a value beyond the operating :speed range of the aircraft. A still further object of the invention is to make it possible to determine that no such critical speed exists within the operating speed range of the aircraft.

We are aware that methods have been heretofore proposed for testing aircraft, but none oi these has been adequate. For example, an article by B. von Schlippe, entitled "Zur Frage derselbsterregten Flugeischwingungen," published in Luftfahrt forschung, vol. 13, No. 2, page 41, de

in a wind tunnel test, is by itself insulclent under most conditions for the determination of the utter speed and especially oi' the utter characteristics of a particular airplane. This method uses artificial excitation at a known frequency. but with an unmeasured source of vibration of unknown power. Further, when using an articialsource oi' excitation, a substantially unvarying power is applied thereto, thus greatly increasing the load on the part when the natural and normal forces to which it is subjected begin to approach the`danger point.

The use of vibration measuring apparatus is. of course, old and although the apparatus described by Draper, Bentley and Willis entitled the MIT Sperry Apparatus for measuring vibration published in the oJurnal of Aeronautical Sciences, 1937, vol. 17, page 281, relates specifically to aircraft vibration, it nevertheless does not recognize the need for equipment capable of measuring flutter characteristics and was, therefore,-not designed nor can i-t be used generally for such purpose. One reason fo'r this is that the equipment for measuring flutter characteristics must be capable of handling ranges of i'requencies and amplitudes entirely different from those for which the special equipment described in this article was designed.

The present invention provides a method which differs in three essential respects from the procedures discussed above, each of these leading to an important increase in the ease andaccuracy of lthe determination of critical flutter speed, or the like,

. One important feature of the invention resides in the fact that the method includes combined ground and flight testing to determine accelerations of a plurality of points simultaneously. More particularly, the method includes the determination on the ground of nodes of the part to be tested. Then in flight the accelerations of nodal and non-nodal points are'simultaneously determined. By properly combining the results, the accelerations which affect the whole part can be distinguished from those which occur within the part, so that the mode of vibration of the part and its flutter characteristics may be more accurately discovered.

A second important feature involves the use of an electrical method. This involves the translation of the corresponding accelerations of different points of the part being tested into electrical impulses, of proportional value and sign, and the combination of these electrical impulses to produce a single signal or indication which will supply to the observer the necessary information as to the final effect of all of the accelerations.

The third important feature is applicable when lsome artificial excitation is used. This is de sirable in order to cause vibrations of the parts so as to render them susceptible of measurement in smooth air, without subjecting the aircraft to the action of gusts, and without relying on uncontrolled gust excitation which is sometimes dangerous. On tle other hand, the superimposed artificial vibrations cannot by the measurement only of the amplitude or acceleration produced determine a `critical speed or provide suillcient data to predictthe critical speed. The present invention, on the other hand, includes the gradual reduction of and accurate measurement of the power which produces the artificial exciting force, in such a manner as to keep the total vibration always below a predetermined danger point. By measuring the power necessary to Fig. 2 is a side View of one of the pick-up units of the apparatus;

Fig. 3 is a plan view of Fig. 2; l

Fig. 4 is an enlarged longitudinal section of the cable used in the cable leads for the pickup device;

Fig. 5 is a cross-sectional view of the pick-up device on the line 5-5 of Fig. 3;

Fig. 6 is a greatly magnified view of the central portion of Fig. 5 showing the details thereof Fig. 7 is a cross-sectional view on the line 1-1 of Fig. 5;

Fig. 8 is a wiring diagram showing the bridge circuits and switches diagrammatically indicated. in Fig. 1;

Fig. 9 is a wiring diagram showing the high frequency amplifier circuit for either the horizontal or vertical channel diagrammatically indicated in Fig. 1;

Fig. 10is a wiring diagram showing the demodulator and the filter circuit for either theA horizontal or vertical channel shown in Fig. 1;

Fig; 11 is a wiring diagram showing the G- meter circuit diagrammatically illustrated in Fig. 1;

Fig. 12 is a wiring diagram showing the oscillator diagrammatically indicated in Fig. 1 for supplying carrier frequency to the apparatus;

Fig. 13 is a wiring diagram showing the power supply circuit diagrammatically illustrated in Y Fig. 1 for supplying power to operate the amproduce a constant acceleration the poweris gradi ually reduced with increase in speed and the critical speed at which the part will begin to flutter to a dangerous extent can be predicted.

This feature is based on the fact that, from the power, the amplitude and the frequency, the damping force can be ascertained; and this damping force reaches zero at the critical speed.

the critical speed can therefore bedetermined.

A fourth feature of the invention lies in the determination of the power required to produce the constant acceleration of the point in question by the measurement of the difference in phase angle between the exciting force and the acceleration.

The method then will provide an occurate predetermination of the critical flutter speed of any Apart of the aircraft.

The invention, while described particularly as useful in the determination of flutter speeds, is also useful forl measurement of other vibratory 4movements occurring in the operation of aircraft,

.relationship of the various parts of the apparatus to each other:

plifiers, and

Fig. 14 is a perspective view of an artificial vibration exciting mechanism;

Fig. 15 is a view of an airplane part showing schematically the use of devicesA of the type shown in Fig. 14;

Fig. 16 is a wiring, diagram of one ofthe devices of Fig. 15;

Figs. 17a to 17h inclusive, are curves illustrating various modes of operation of the apparatus;

Fig. 18 is a View showing schematically the use of pick-up devices on a whole wing of an aircraft.

In Fig. 1, the pick-up units are indicated at A1, A2 A16. These units are placed upon the structure to be analyzed, and when the various structural parts to which they are xed vibrate,

By extrapolatmg one or more of these variables* 50 the accelerations cause a modification in the elecoscilloscope or oscillograph.

The pick-up unit which is referred to above is a mechanism -by which accelerations imposed thereon produce a relative movement between an inductor and the frame of the pick-up unit which in turn causes changes in the inductances of two built-in coils; the inductances of one coil being increased as that of the other is decreased and vice versa. By means of additional equipment connected in a 'suitable electric circuit these changes in the inductance of the pick-up coils are effective in varying an electric voltage, with an associated electricY current, in a manner representative of the initial mechanical acceleration of the structural part being analyzed.

Referring back to Fig. 1, the relation of the pick-up units to the general circuit becomes more centralized measuring equipment, connection cables '10 feet long having been used. There is no practical limit .to the length of cable which may be used. Housed in the centralized measuring equipment and corresponding to each pick-up unit is an electrical bridge circuit, which will be more fully described with reference to Fig. 8. Alternating current at audio-frequency is supplied to the bridge circuits by an output oscillator. This impressed frequency is referred to as the "carrien" 'I'he output voltage oi' each bridge circuit is determined by the voltage of the impressed carrier and by the degree of balance of the bridge circuit. This factor of bridge balance is directly affected by the position within the pick-up of its inductor. which in turn is controlled by the mechanical accelerations being imposed on an'. individual pick-up unit. Therefore the output voltage of each bridge circuit will consist of a carrier frequency component Vwhich is modulated in accordance with the mechanical acceleration to be measured.

-T'he voltage output from the bridge circuit is amplified at one or the other of the two high frequency amplifiers." Beginning with these amplifiers, it will be noted from Fig. 1 that there are two duplicate sets of equipment forming two separate and independent channels which feed the oscilloscope. One istermed the "vertical" channel because it controls the vertical set of deflection plates in the cathode ray tube, while the other is termed the horizontal channel because it controls the horizontal set of deecting plates. l

The unit which contains the sixteen separate bridge circuits is provided with suitable switches so that the output from any one oi the pick-ups may be fed into either ofthe amplifier channels. In addition to the selective operation of any lpick-up unit into either channel, the switching facilities also make it possible to secure `the following results by direct observation:

(a) Determine the vector sum oi' two or more pick-up accelerations directly combined and fed into one or the other of the amplifier channels.

(b) Under (a) above,l the phase of any of the pick-up outputs may be changed by 180 so that the vector differences are obtained. i

(c) By using both amplier channels simultaneously, the phase relationship between any two pick-up accelerations, of the same frequency, may be determined.

(d) The methods o f (a) or (b) above may be Ibe somewhat unconventional in appearance because of the presence of the carrier frequency.-

The film speed would be adjusted to a value which would bring out the shape of the vibration phenomena. Since these phenomena-are of relatively low frequency as compared to the carrier, it follows that the film movement would be too slow to resolve the transverse motions of the light beam at mcarrier frequency into separate traces on the film. Therefore a solid or. block pattern would result. The fixed amount of unbalance in the pick-up bridge circuit would lead to an oscillograph input of carrier frequency current having a constant maximum value. This would produce a solid block trace on the film of constant amplitude or height. Ii' the amount of unbalance in the bridge circuit is varied by the pick-up in response to imposed mechanical acceleratlon, the amplitude of the block trace will be varied as the film moves along so that the profile' of the trace will show the mechanical acceleration.

In addition to the provision for obtaining photographic records from an oscillograph, the apparatus permits the obtaining of immediate visual indications of the accelerations expressed by the pick-up units. are of the greatest importance in making flutter tests of aircraft where the information is needed before time can be taken to develop film from the oscillograph. Two kinds of visual indicators are used as shown in Fig. 1. One is a conventional oscilloscope knowny conventionally as a cathode ray oscilloscope. and the other is a DArsonval indicating instrument which comprises a D. C. microammeter connected to a special circuit in such a way that its indications are approximately proportional to the average accelerations, or '-g" which is being imposed on the ended upon Fig. 1.

combined with (c) to determine the phase rela-V Y tionship existing between the vector combination of acceleration expressed by one group of pickups andthe output from a separate single pickup or from a group of pick-ups.

As described above, the two output circuits of the bridge and switching mechanism are connect- V ed to the horizontal and vertical high frequency amplifiers. The high frequency designation is used because the voltages to be amplified are all at carrier frequency, or, to be exact, they occupy the band between the carrierv frequency plus vibration frequency, and carrier frequency minus vibration frequency. This. as will be explained later, permits important simpliflcationsand economies in the construction of the ampliiier.

Each of these amplifiers contains two gain stages which build up the voltages. followed kby a power output stage of amplification. The

power output vstage is intended to operate anl electromagnetic o'scillograph to obtain vibration records on film if desired. These records would connected pick-up unit. Accordingly, this latter instrument is termed a G-meter as leg- In Fig. 1, a selector switch labeled G-switch"is used to connect the G-meter to either the horizontal or vertical channel. Figa/1 further shows a demodulator, filter, and low frequency amplifier interposed between each high frequency amplifier and the deflecting plates of the cathode ray tube in the oscilloscope.

In the demodulator, the carrier frequency is sup..

pressed and the modulation components representing the original mechanical vibrations are passed on to the filter. Demodulation serves two purposes: (a) It leads to a single line pattern on the oscilloscope rather than al block pattern of the type disclosed in connection with the electromagnetic oscillograph; and (b), demodulation also permits the use of a filter.

The output. of the demodulator contains the electrical representation of all the mechanical accelerations falling within the range,'and directed along the sensitiveaxis of the particular pick-up being analyzed. These accelerations may be so numerous and of such widely difl'erent frequencies that a simultaneous `viewing of them all on the oscilloscope screen would be very confusing. For example, in making flutter measurements in an aircraft, the presence of high frequency accelerations due-to engines, for example, would be superfluous and objectionable. The filter is therefore included. This unit readily passes cn the low frequency phenomena which are important in making a flutter investigation, but the high frequency phenomena are so strongly attenuated that they are rendered negligible. The filter thus described and as used is a lowvl pass lter. However, a high pass filter, or a band rejection filter could be used as well. A high pass filter would be appropriate if it were desired to eliminate indications due to low frequency phenomena and to focus attention on high frequency phenomena. A band pass filter has both an upper and a lower frequency linut, making it possible to concentrate attention on the phenomena occurring when frequencies lie between the upper and lower limits of the band pass filter, high and lower frequencies outside the limits of the filter being suppressed. The band rejection filter is exactly the converse of the band pass'fllter as it has both high and low frequency limits, and frequencies falling within these limits are suppressed, while all other frequencies are passed.

The output of the filter goes through a low `frequency amplifier before being applied to the defiecting plates of the oscilloscope. This low frequency amplifier is used to obtain a. voltage suflicient to operate the oscilloscope properly without the necessity of working with excessively high voltages in the demodulator and filter units. However, the gain in the low frequency amplifier is held to the necessary minimum because of the difficulty of extending the region of uniform gain into the region of low vibration frequencies which the equipment is intended to cover. Amplification Without discrimination against the extremely low frequencies, in the range of 60 to 600 cycles perI minute is readily accomplished in the high frequency amplifier, but after the demodulator has removed. the carrier frequency.

any further amplification must cover the entire range of the mechanical vibration frequencies which the equipment is being used to measure. Because of this necessity for covering a range of frequencies which includes very low frequencies, it follows that for a given amount of amplification, a low frequency amplifier would be more expensive and also more bulky than a high fre- -quency amplifier for the modulated carrier current which would give an equal amount of gain.

The oscilloscope unit is of the conventional type and is equipped with a variable frequency saw-tooth oscillator which can be switched onto the horizontal deiiecting plates "of the cathode ray tube to provide a means of obtaining astationary trace on the projection screen of the oscilloscope. This can be done only for recurrent phenomena, and the necessary condition for a stationary trace is 'that the saw-tooth oscillator be adjusted to the same frequency as that of the phenomena which are affecting the vertical deflecting plates of the cathode ray tube. This effect is used as a means of evaluating the frequency of the observed phenomena. The frequency control of the s'aw tooth oscillator is calibrated in terms of cycles per minute so that as. soon as it has been adjusted to obtain a stationary trace on the screen, observation of the control setting gives a direct reading of the frequency.'

In addition, the oscilloscope is equipped with two built-in amplifiers, each of which is provided with a gain control so that the amplification factor may be varied from zero to maximum. One of these amplifiers operates in the vertical channel and the other in the horizontal channel. This is a conventional arrangement in standard Oscilloscopes, but in the instant apparatus the arrangement is special in that the amplifiers are designed tc operate at very low frequencies. Fur-l thermore, these amplifiers may be cut out of the circuit so thats direct connection from the low erated from a common member the vibrations frequency amplifier to the deflection plates of the cathode ray tube may be obtained. This does away with any low frequency limitation to the oscilloscope amplifiers. However, for normal operation, their low frequency characteristic is satisfactory, and in normal operation these amnected. This unit requires a source of electric power for its operation as well as do the amplifiers and demodulator units. As the apparatus is primarily intended for use in an aircraft while the aircraft is in flight, arrangements are made for the supply of all necessary power from storage batteries which may or may not be connected to the generators and electrical system of the aircraft. The power supply is therefore continuous current at 12 volts. 24 volts may be used as will be later explained in the description of the oscillator and power supply units.

In order to provide the high voltage continuous current required by the amplifiers, one of the auxiliary units is necessarily the power supply unit indicated in Fig. l. This unit contains the dynamomotors used to secure the high voltage, and the filter equipment for the purpose of removing the ripple voltages produced by commutation. Ihe oscillator and power supply are opbattery which therefore supplies all the equipment except the oscilloscope and the oscillograph.

' In the use of a conventional oscilloscope with the apparatus, A. C. at volts, 60 cycles must be supplied. As shown in Fig. 1, a vibrator power supply is connected to the battery feeding the oscilloscope and oscillograph. This vibrator power supply is also a commercial and conventional piece of equipment and is used to convert low voltage D. C. battery power into 110 volts, 60 cycles A. C. Of course the electrical system of the aircraft may be substituted for the battery. Use of this second battery helps to reduce the drain on the battery feeding the oscillator and power supply previously described, and also prevents disturbances associated with the vibrator from affecting the oscillator and amplifiers. This battery can also be used for operating the electromagnetic oscillograph.

In Figs. 2 to 7, the structure of the pick-up unit is shown. A non-magnetic casing 2 encloses the operating mechanism, said case having a base l which is adapted to be bolted, or screwed, to a of which are to be studied. From a protecting cap i bolted at B to casing 2, tube Il! extends and forms a protective conduit for cable I2 which is composed of three insulated wires 10, 12, and 14, surrounded by a wire braid Il, Fig. 4, over which lies another insulating coat I6. Clip I3 holds cable l2 against movement lwithin tube i0. The wire braid shields thecable against any inductances which could influence the electrical currents passing from the pick-up device tothe switching apparatus. Each pickup is located at a. different position in an aircraft to be tested. 'The wires in cable i2 are joined to connecting leads which extend to the lcentral mass by a pin 32.

2,305,268 lswitching mechanism placed in a central location,

as in the cabin of an aircraft. When cable l2 is connected to the leads to the switching mechanism, a slidable shield I8 consisting of metallic braid is slid over the joint, thus shielding the joint against outside influences. After the pickup has been installed, the metallic braid I4 is grounded to the structure being tested, thus increasing the effectiveness of the shield.

The details of the pick-up are more fully described in Figs. 5, 6 and 7. In Fig. 5, a ringshaped ferrous mass 20 having the form of an I in cross-section, has a circular metallic disc 22 fitted to each side thereof. This mass lies inwardly of brass ring2`4. The open ends of ring 24 are closed by copper beryllium spring discs 26, against which the central portion 28 of mass 20 bears. Secured to mass 20 on the outer faces of the discs 26 are second metallic masses 30 which function as poles. These poles are secured to the Second brass rings 36 lie above and below the central ring 24, and clamp the peripheries of discs 26 between rings' 24 and 3G. Rings 36 have inwardly directed anges 38. Phenol bre spools 40, on lwhich are wound coils 42 are held against flanges 38 by plates 44 and 46, which cover the top and bottom of this assembly, these plates being clamped together by bolts 48 placed outwardly of rings 24 and 36. Plates 44 and 46, respectively, carry poles 50 threaded into projections on the plates, poles 50 being adjustable to or from masses 30. Mass 28 lies within the flux field of coils 42'. Poles 50, after being adjusted, are locked in position by screws 52. Plate 44 is of suiiicient diameter to extend to the outer edge of a securing flange 53 surrounding casing 2 whereby the plate is secured to casing 2 by bolts 8 which also secure the cap 6 to casing 2. Between plate 44 and cap 6 is located an insulating member 54 which supports binding posts 56. Leads 51 from coils 42 are secured to these binding posts.

Ring 24 is drilled at 60, and 36 is drilled at 62 to provide fluid passages. After the device is assembled, it is evacuated and filled with a noncompressible :duid Awith a nearly constant viscosity over the operating temperature range by a convenient means as through a plug in the wall of casing 2, not shown. As more clearly shown in Fig. 6, spring discs 26 are slightly deiiected from their normal position by the central portion 28 of mass 20. 'I'hat is, the discs are constructed as being fiat, but in assembling the pick-up, the central portion of the discs is deilected approximately 0.002 inch. 'I'his initial straining of the discs is considered important. As the portion 28 of mass 20 moves in response to accelerations, an inaccurate response to vibrations of low magnitude would occur if both discs passed through their central flat position at the same time, and this would be particularly true for accelerations of low magnitude. By initially placing discs 26 under tension, and concave toward each other, the two discs never pass through their center, neutral, or iiat position at the same time, inasmuch as the mass 20 must deflect several thousandths of an inch before either disc can pass through a at condition.

The operation of the pick-up device to measure accelerations is as follows:

An alternating current functioning as a carrier current is supplied from the oscillator diagrammatically indicated in Fig. 1 to coils 42, which are connected in series opposition through leads 58 to cable l2. Before iinal assembly of the unit, pole pieces 50 are adjusted and locked in place in order to set the proper air gap between them and poles 30, this setting the amount of inductance that can be obtained in coils 42. As a mass 30 approaches an adjacent pole 50 on one side of mass 20, the corresponding poles on the opposite side of the mass become further apart. Changes in the respective air gaps be-4 tween adjacent masses 30 and poles 50 cause a change in the reluctance of a magnetic circuit existing between the coils 42 and the mass 20, and a corresponding change in the inductance of each of the two coils.

The inductive resistance of any coil is equal to the product of its inductance by 2 1r times the frequency of the alternating voltage applied.

The inductance is proportional to the amountof flux produced by a given current in the magnetic circuit associated with the particular coil, and this ux in turn is inversely proportional to the reluctance of the magnetic circuit. Therefore it is apparent that the motion of the inductor relative tothe frame (in response to acceleration imposed on the pick-up) results in changes in the inductance and reactance of both the upper and lower coils. These changes occur in such a manner that as one coil has its reactance increased by the approach of the inductor, the other coil is experiencing a decrease in inductance (and reactance) due to the increasing separation from the inductor.

The provision for adjustment of the pole pieces serves two purposes. First it provides a means of making the 'reactanccs of the two coils equal. 'I'his is of considerable importance in securing a 1:1 balanced bridge circuit for convenience in switching and combining indications as will be discussed later. Secondly, the pole piece adjustment provides a means of varying the sensitivity. The maximum pick-up sensitivity occurs for a small gap separation, but when the pick-up is to be subjected to large values of acceleration which would produce correspondingly large values of inductor displacement it is necessary to increase the gap separation in order to avoid actual striking of the pole pieces.

Thus, an alternating modulation of the carrier frequency primarily supplied to the coils 42 ls produced. After adjustment, the assembly is completed, dampening fluid placed in the unit, and the unit is then secured to a structural part vWhose vibratory motions are to be studied. Movement of the structural part creates movement in the unit because of the inertia of mass 20. This movement of mass 28 is damped by the flow of the fluid which must pass through ports 60 and 62, respectively, as the spring discs 26 aredeiiected.

The use of two diaphragm springs 26 with precisely fixed radial location is very important because it very effectively provides selective respense. That is, the only motion possible between the inductor and the surrounding frame is pure axial displacement. Therefore, of all the .random accelerations which may be imposed on the pickup, vonly those components of acceleration which are directed along the axis of the unit will be effective in producing motion of the inductor and response of the entire, equipment. It

is obvious that this property of selective response y is indispensable in making any analysis of an unknown and complex motion.

The use of the fluid not only prevents excessive amplitudes of movement of the mass, but

prevents the inductor system from vibrating at its natural resonant frequency in response to any random or shock excitation which may be imposed upon the pick-up. Again, large amplitudes of spring deflection are eliminated which would lead to the failure of the springs from fatigue.

.The range of frequencies over which the inductor type pick-up has been experimentally used extends from about 30 cycles per minute up to approximately 4500 cycles per minute. These figures, however, do not constitute either an upper or lower limit of usefulness. The inductor pick-up is designed to operate as an accelerometer; that is, the displacement of the inductor relative to the frame of the pick-up is designed to be proportional tol the acceleration which is imposed on the pick-up as a unit.

Deviations from this ideal characteristic become more and more pronounced as the frequency of the imposed mechanical vibration is increased from`zero frequency toward the natural resonant frequency of the inductor, (or moving mass which is supported on a system of springs within the pick-up) Theory indicates however that the use of viscous damping amounting to 62% of the critical value will result in a deviation from the ideal characteristic ofless than i3% for any frequency within the range from zero to 75% of the natural resonant frequency of the inductor system. The upper frequency limit of pick-up to give a useful response, a great deal will depend upon whether sensitivity to displacement is intended, or sensitivity to acceleration. In principle the response of the entire equipment to acceleration can be made essentially uniform `over a frequency range extending from any low usefulness is therefore related to the natural resonant frequency of the inductor system. For the pick-up unitsy actually in use, this natural frequency was designed to be 12,000 cycles per minute. In principle the upper limit can be extended almost indenitely by the use of stiff springs and a small mass. On this basis the theoretical ultimate upper frequency limit of pick-up usefulness should not be less than 1,000,000 cycles per minute. On the other extreme there is no theoretical lower limit of frequency response for the accelerometer.

However, because of the fact that the sensitivity of an accelerometer varies inversely as the square of the natural frequency, this consideration of sensitivity usually sets a practical upper limit for the natural frequency of the moving system (i. e., the inductor system in the case of the subject pick-up). Considering only the response to accelerations, of the pick-up unit per se, practice coincides with theory and there is no low frequency limitation. However, because of the electrical amplifiers used in the present equipment, the entire arrangement of equipment doesbegin to lose sensitivity at frequencies lower than 60 per minute although good response is available even. down to, and somewhat below 30 per minute.

The preceding discussion of sensitivity is on the basis of response to acceleration. In normal usage these accelerations will be produced by periodic displacements (of the structure or part lto which the pick-up is attached) from a neutral or at rest position. In this connection, since thel maximumyalue of acceleration associated with a sinusoidal motion is proportional to the product of the maximum amplitude of displacement and the square of the frequency it follows that the sensitivity of even an ideal accelerometer device to periodic displacements is inversely proportional to the square of the frequency. Hence. if the useful range of frequency" for the subject pick-up and equipment is discussed from the standpoint of possessing suicient sensitivity frequency other than zero up to at least 1,000,000 cycles per minute. 'Howeven due to the inherent characteristics of even an ideal accelerometer, the response as related to periodic displacements will be very great at high frequency decreasing to zero to zero frequency.

There is another factor, which is very elementary, but absolutely basic in any practical discussion-of the useful range covered by the pick-up and apparatus. The apparatus will be useful in observing and measuring a given vibration only when the frequency lies within the working range, and also when the magnitude of the accelerations associated with the vibration is suiiicient to produce a proper response of the apparatus, without being so great as to damage the apparatus.4 This question of combining sufllcient sensitivity with adequate ruggedness is of more practical than theoretical importance. In point of fact, the apparatus is so rugged that pick-up units tested under severe vibration amounting to over 100 times the acceleration of gravity were undamaged. At the same time sensitivity is such that 0.02 times the acceleration of gravity is suilicient to give usable indications.

Fig. 8 illustrates the wiring system for thel bridge circuits and switches shown in Fig. l. A transformer 1G, which is in` the output circuit of the oscillator, Fig. 1, and which is supplied with current in a manner to be described below, in connection with Fig. 12, represents the source of carrier frequency current to the bridge circuit. The exact value of carrier frequency voltage applied to the bridge circuit is important because the output voltage of the bridge is proportional to the product of the input voltage and the degree of unbalance present in the bridge. Therefore, two potentiometers marked Co for coarse and Fi for fine are shown in Fig. 8 connected to leads 11a and 11b from the transformer 18. These two potentiometers enable accurate adjustment to be made, while the volt meter 18 gives the actual value of the voltage applied. Leads 11a and 11b, the Co and Fi potentiometers, and volt meter 18 are common to all pick-up units A1 to A13, inclusive, Fig. 1. Individual circuits for but two units-are shown for purposes of exampleinFig. 8.

A switch connects the bridge circuit for one pick-up unit, for example unit A1 of Fig. l, to leads 11a and 11b, and makes or breaks contact between the bridge circuit and the oscillator voltage. Switch 82 is a reversing switch for altering the phase of the carrier input by 180. Switch 84 is a double pole, double throw, center off, three position switch connecting the bridge to transformer B6, composed of parts 86a and 88h. Thus the switch connects the output oi the bridge through leads a to the vertical channel high frequency amplifier through transformer 88a, or to the horizontal channel through lead: 85h and transformer lib, or isolates the bridg output from both channels.

The bridge proper has two ratio arms with s single common adjustment constituting the potentiometer 88. Fixed resistors 90 and $2 comprise a large percentage of the resistances i1 each arm to make the resistance in each arn constant. and thereby reduce the delicacy of thl sistance.

isapplied to the resistors 96 and 98 adjustment required in the potentiometer 88 in order to secure a balance of the circuit. An individual piek-up is represented by two inductances such as 95 and 91 which form the principal parts of the two variable arms. These inductances are, of course, the two coils 42 built into the pickup 2, and connected to the-bridge circuit through the leads 10, 12 and 14.

The movable pole pieces described in connection with the pick-up unit 2 are adjusted so that the inductance of the lower coil exactly matches that of the upper coil when the pick-up unit is at rest. Subsequently in balancing the bridge circuit, the potentiometer 88 is adjusted so that the 1:1 inductance balance in the pick-up is matched by a 1:1 resistance balance of the ratio arms. This condition is termed a magnitude balance of the bridge circuit but does not represent a completely balanced condition which would correspond to a no voltage output regardless of the amount of input voltage applied. The remaining balance condition is termed phase balance. The two balance requirements, "magnitude and phase, arise from the fact that the pick-up coils function in the electric circuit both as inductances and pure resistances, and neither solely as a pure inductance or a pure re- The inductance eilect is due to the magnetic ilux which links each coil 42, and the resistance eiect is due partly to the ohmic resistance of the copper-Wire used in the windings and leads, and partly to the losses which arise from the reversals, or alternations of the magnetic ux occurring in response to the alternating current applied to the bridge circuit.

The inductive and magnetic circuits used for both upper and lower coils in each pick-up 2 are made as nearly identical as possible. In fact, the magnetic circuits are adjusted by means of the movable pole pieces 50 so that the inductances of the coil are identical. No means, however, is built into the pick-up unitfor making the loss or resistance eilects of the two coils absolutely identical. Such is .accomplished by means of a variable resistor or phase balance control 94 which is in series with one of the two pick-up coils. By experiment the proper connections of the pick-ups are determined so that this phase balance control 94 wil1 be in series with the coil of the pick-up having the smaller eiective resistance. The resistance introduced at 94 is adjusted until it is just suicient to make up the deficiency existing in the low resistance coil. Thus by means of two adjustments, the pick-up coils are simultaneously brought into a 1:1 relationship with each other, and both their phase or resistance components and their magnitude or inductance components are made identical to produce a Vcomplete balance-of the bridge circuit with the corresponding adjustment of the ratio arms into a 1:1 relationship. m

This balance condition holds only so long as the pick-up unit is at rest. As soon as the pickup unit experiences an acceleration which moves the inductor mass 2U, the balance between the coils is disturbed and their inductance and therefore their reactance are no longer the same. Consequently, a voltage difference appears between the midpoint of potentiometer Il and common lead 12 of the two coils. This voltage connected in series. Resistors 96 and 98 constitute, in eeet, a potentiometer, and the voltage across resistor 96 is applied through switch 84 to transformers 86a or 86h which iced the high frequency amtil Vsistor 98 Awhich just equalsl the resistance of resistor 96, 'transformer 86 receives only half of the unbalance voltage which 7 pliers. By adjusting resistor 98 to zero resistance the total amount of unbalance voltage is applied to transformer 86, but when each remay be adjusted to its maximum value is developed. Hence resistor 98 serves as a sensitivity control" for varying thebridge output for a given acceleration over a range of 2:'1.' i

The bridge circuit fora single pick-up 2 has just been described. In Fig.'8 the bridge circuit for a second similar pick-up isalso shown. It is identical with that for the rst circuit just de scribed and the corresponding elements have like reference characters with the subscript a. Leads 11a and 11b from the transformer 16 are commonto the bridge circuits for each individual pick-up. An apparatus which has been actually used has contained a total of sixteen pick-ups and associated bridge circuits. Of course, the number of pick-up units with their corresponding `bridge circuits may be varied without limit. As

the description of one bridge circuit serves for any number of identical bridge circuits, no further bridge circuits are described. It-is noted, however, that the proximity of the various components of a plurality of circuits, as well as of them'res, would provide capacity coupling between the circuits and lead to objectional interaction therebetween if very thorough shielding were not employed. Consequently, metal panels and boxes are used to house the apparatus, and the components of each individual bridge circuit are grouped as compactly as possible and are surrounded by metallic shields from adjacent circuits. Shielded lead wires, connection with metallic bodies, blocks, and connection cable with metallic shield braid, also contribute to the thorough isolation of the individual circuits.

All of the above mentioned shields are grounded to the metal boxes housing the equipment, all of which are connected together, and,L

of course, each and every component and Wire comprising a bridge circuit has capacityI to ground. These capacities are unavoidable, but nevertheless they represent parasitic and undesii-able effects. However, these eiects are made negligible by the inclusion in the circuit of a socalled Wagner ground. This grounding device 1s composed of two resistances |00, l02 connected across leads 11a and 11b near the volt meter 18. and grounded at l 04. These resistances are made equal to each other and are of low impedance as compared to the various arms of the bridge circuits. Because all bridge eircuits are balanced to a 1:1 condition, this single Wagner ground is also adjusted to a. 1:1 relationship, and is thus able to serve all bridge circuits.

This Wagner eects due to grounding in the following manner:

,With a given pickup un'it at rest and its bridge circuit balanced. the potential of the mid-point of the potentiometer 8l is the same as the potential of the lead-in 12, because each of these two points is at a potential midway between that of the two wires leading from switch 82. Another point which occupies a similar mid-position and therefore is 'at thesamepotential, is the point midway between the Wagner ground arms |00, |82. But, this point is actually grounded and hence the two other mid-points whi h are at this same potential are also at ground potential, although they are not actually grounded. Since ground eliminates undesirable one or the other of the Wagner ground arms |00,

|02. Since all stray ground capacities are actually of small magnitude, it follows that their impedances are very high and therefore have negligible effect on the total impedance when considered as operating in parallel with a low impedance circuit member such as |00, |02.

How various pick-up units can be combined by the switching arrangements shown, for the purpose of analyzing various vibrations, is now described. As previously stated, it is possible to accomplish vector addition, vector subtraction, and to establish phase relationship of the mechanical accelerations to which two or more of the pick-up units are responding. Vector addition of the accelerations to which the two pickups corresponding to the bridge circuits in Fig. 8-

are responding, can be made by closing switches and 00a and also closing switches 82 and 82a, both switches being closed in the same direction.

Switches 8| and 84a would each be connected to'v leads 05h for the horizontal channel, or, if desired, to leads a for the vertical channel; that is, to either transformer 86h or 86a. 'I'he effect of this switching combination is-to form what l amounts to a single bridge circuit out of the two individual circuits. Each arm or member of the resultant bridge is formed by the parallel combination of the corresponding members of the two individual bridges. Thus, resistor 96a is directly in parallel with 96 while the ratio arms are also parallel directly. The combination of resistors and inductance 94, and 98 is parallel with the combination of resistors and inductances 94a, 95a. and 98a.

The mathematical equations which describe the output of this combination in terms of the accelerations imposed on the two pick-up units shown are complicated, but .the net result is identical with that obtained for the description of the carbon pile pick-up in application Serial No. 214,562, in which the carbon pile pick-up had a bridge circuit similar to a simple Wheatstone net operating on continuous current from a battery, rather than on alternating current operating from an oscillator. This result briefiy means that the electrical output of the combination bridge circuit is equivalent .to that which would be obtained from a single pick-up unit and bridge circuit if the single unit were subjected to one-half of the vector sum of the accelerations being imposed on the two pick-up units of the combination.

The vector form of the accelerations which is mentioned above must be understood as referring to accelerations which are associated with periodic motions and which are therefore themselves periodic. Such a quantity is similar to ,an alternating current or voltage, and may, in a similar manner, be represented by a vector which possesses an effective value such a", a magnitude, and which also acts with a certain time or phase displacement when referred to a time reference standard. Therefore, in speaking of the vector sum of two such accelerations. it is clear that the direction part of each vector is actually a time phase, and does not refer to the physical'direction or orientation in space along which the accelerations are acting. It is recognized that instantaneous values of accelerations acting with a given orientation in space are vectors, but the vector combination accomplished by the instant invention is concerned with time phase, and not with space orientation. This does not mean that the space orientation is indefinite or indeterminate as far as the apparatus of this invention is concerned. There is no ambiguity, because the basic mechanical system of the pick-up umts is selective in its response, and only those accelerations which are directed along the axis of the pick-up are effective in producing any electrical response in the equipment. In view of the above, it is clear that a vector combination accomplished by this apparatus is significant only when the accelerations combined are both periodic and both of the same frequency, because it is only when the frequencies coincide that there is any meaning attached to the conception of a time phase displacement existing between two vectors representing periodic accelerations.

With this understanding of the terms used, the possibility for use of vector combinations by the apparatus will be discussed. Referring to the previous description of the switching combination required in order to obtain the vector addition, it will be noted that the essential conditions for vector additions are:

( a) Both switches 80 closed; (b) Both switches 82 closed'in the same direction; (c) Both switches 84 andv 84a closed to make connection to the same high frequency amplifier of either the horizontal or vertical channel.

It is to be noted that the output of this combination is one-half of the vector sum directly. Furthermore, the method is general, and, if desired, the vector sum of accelerations applied to all the pick-ups could be obtained. When such is desired, all the switches areclosed as set forth above. The output of this combination will be the vector sum of the several accelerations to which the individual pick-up units are responding, divided by the number of pick-up units entering into the combination. The fact that the sum is always divided by the number of pick-ups is an inherent characteristic of this method of switching, and means that basically the action is that of averaging. Thus, if the accelerations entering into a given result are al1 in phase, the vector sum becomes a simple arithmetic sum, and the act of dividing between the number of units in use means that the result is the average of the individual accelerations.

Vector subtraction is, of course, only a special case of vector addition. Subtraction implies that the sense of the vector is reversed, for example, its phase is shifted by l80. This is readily accomplished by use of the switches 82 and 82a, which reverse the phase of the input or carrier frequency voltage to the bridge circuit. Phase reversal of the input voltage results in a reversal or a phase shift of the bridge output voltage. The switching arrangement for subtraction of the pick-up unit connected through switch V added to the response of the second for addition, the output one-half arising from the "a, and the pick-up unit connected by switch lli, is asfollow (a) Switches 80 and 80a closed;

(b) Switch 82 closed in one direction, and switch 82a closed in an opposite direction;

(c) Switches 84 and 84a closed in the same direction.

'I'he resulting output of this combination will be one=half thevector dilerence; that is, onehalf the vector sum of one pick-up response pick-up taken after a phase shift of 180. Actually, the resulting output would be essentially the same if under (b) above, the positions of both switches 82 and l82a were reversed. The essential point is that the switches 82 and 82a are all closed in the same direction for addition, while they are closed in opposite directions for subtraction. Just as suit divided by the total number of pick-up units connected to the particular amplifier channel.

An example of a practical useiof the adding and subtracting features of the equipment is given in separating the bending and torsion modes of vibration in an airplane wing, or tail surface. 'Ifwo pick-up units are attached'to the structure of the wing, both at the same distance out from .the Unit is located forward of the elastic'axis, and the other aft of this axis. The elastic axis is by denition the axis about which torsion takes place, hence the axis itself offers no deflection due to torsio 'I'hat is, the motion at this point in the wing cross-section would therefore consist solely of pure bending motion. Both pick-up units are similarly oriented with their sensitive axes directed to respond to motion due to wing bending. With the two pick-up circuits connected to the vertical channel and combined for subtraction, the resulting trace on the oscilloscope screen is used to nd the vibration frequency, and the height of the trace on the screen interpreted in terms of the calibration of the apparatus indicates one-half of the acceleration due to torsion.

The accelerations due to bending would be the same at each pick-up unit and the subtraction arrangement therefore cancels these accelerations. When using the accelerometer characteristic of the pick-up, thel actual diiference in pickup motions for the two units is given by the accelerationsas found by one-half the square of the frequency; `this one-half factor being introduced to cancel the subtraction characteristics of the equipment. This quotient, giving the 'difference in the linear motion in the two pickup units due to torsion, is further divided between linear distance by which the pick-ups are separated in their attachment to the wing structure, to wing torsion in radians.

'Ihe determination of the pure bending motion can also be ascertained if the position of the elastic axis is known, so that the'pick-up units can be located at equal distances from it; one being forward of the axis andthe other being aft. If this condition is met, then the pick-up circuits are connected for addition, with the result being the average of the two accelerations, which represents that of the mid-point ofthe line connecting the two pick-ups.' that is, the acceleration occurring at theelastic aids. 'I'his point by deiinition does not partake of any torsional motion, and hence the resulting trace on the oscilloscope from the oscilloscope dividedV and the resulting quotient gives the angle determination and evaluation of the wing motion consists of the vector recenter line of the airplane. One

stationary ligure. If

'celerations occurring at the span location of the pick-up, that is, at its distance from the center line of the airplane.

In the entire foregoing discussion of vector addition and subtraction combinations for two or more pick-up units, equal sensitivity has been assumed for all the units and bridge circuits eni tering into the combination. This condition is most important, in fact,.it is absolutely essential to the successful use of the vector combination feature. All bridge circuits and pick-up units are made as nearly identical as possible. In this connection the adjustable pole pieces of the pick-up units are very important. However, in spite of careful construction, vsome diierence may be found in the response of two bridge circuits even when their pick-up units are subjected to' the same acceleration. This condition is com pensated, however, by proper adjustment of the resistors 98and 98a, Fig. in each bridge circuit. These resistors in combination with the unadjustable resistors 96 and 96a form a potentiometer by which the output of each bridge may be adjusted without alteration in the phase of the output voltage. Thus the overall sensitivity of each pick-up unit and .tion of the switches would be used toobtain an indication of the phase relationship:

(a) Switches I0 and 80a closed;

(b) Switches 82 and 82a closed in the .same direction;

(c) Switch 84 closed to make contact with the vertical channel, and switch 84a closed to make contact with the horizontal channel.

The. spot on'the oscilloscope screen will trace a pattern in which the vertical components of motion represent acceleration imposed on the rst pick-up through switch 8| while the horimntal components of the motion are due to acimposed onv the second pick-up through switch 84a. For recurring periodic vibrations, the trace on the screen will be a closed the vibration is all taking place at a. single frequency, the ligure on the screen will be some form of an ellipse. For zero and 180 degree phase displacements. the ellipse degenerates into a straight line, which will lie in the rst and third quadrants for zero degrees, and in the second and fourth quadrants for 180 degree displacement. For and 270 degree displacements, the ellipse becomes a circle, pro-l tor from observations on the oscilloscope screen and magnitude of In connection with the switch lmsltion'used` as to the shape, orientation, the ellipse.

to secure phase above, that the patterns, it will be noted from (b) switches l2 and 82a may be closed 8, 4which are included of a structure which is vibrat- 

